Large area wireless CDMA system and method

ABSTRACT

A transmitter and a receiver using a set of Large-Area Code-Division-Multiple-Access (LA-CDMA) codes. The LA-CDMA codes are generated from the steps of generating a plurality of pluses, and generating a plurality of codewords. The plurality of pluses has a plurality of intervals between each of the pulses in the plurality of pulses, respectively. Each pulse of the plurality of pulses has an amplitude and a polarity. Each interval of the plurality of intervals is unequal to other intervals of the plurality of intervals. Only one interval of the plurality of intervals is an odd number larger than a value of a smallest interval of the plurality of intervals. No value of any interval in the plurality of intervals is a sum of any two or more values of two or more intervals, respectively, in the plurality of intervals. Each codeword in the plurality of codewords is different from other codewords in the plurality of codewords, by assigning, for each respective codeword in the plurality of codewords, a polarity to each pulse in the plurality of pulses. The plurality of codewords are either orthogonal, or bi-orthogonal, or trans-orthogonal. The codewords are generated such that a cross-correlation between any two codewords in the plurality of codewords has side-lobes with any of the values of zero, plus amplitude squared, and minus amplitude squared. Any codeword in the plurality of codewords has a zero correlation window in an auto-correlation function and the cross-correlation function with a magnitude equal to the amplitude squared and with a width equal to two times the smallest interval of the plurality of intervals.

RELATED PATENTS

[0001] This patent stems from a continuation-in-part patent applicationof U.S. patent application Ser. No. 09/501,666, filed Feb. 10, 2000,entitled A SCHEME FOR SPREAD-SPECTRUM MULTIPLE ACCESS CODING, whichstemmed from Patent Cooperation Treaty (PCT) patent application no.PCT/CN98/00151. The benefits of the earlier filing date of the parentU.S. patent application and PCT patent application are claimed forcommon subject matter pursuant to 35 U.S.C. §§119, 120 and 365.

BACKGROUND OF THE INVENTION

[0002] This invention relates to spread-spectrum communications, andmore particularly to a coding technique for a large area spread-spectrumCDMA system.

DESCRIPTION OF THE RELEVANT ART

[0003] The growing popularity of personal communication services coupledwith the scarcity of radio bandwidth resources has resulted in theever-increasing demand for higher spectral efficiency in wirelesscommunications. Spectral efficiency refers to the maximum number ofsubscribers that can be supported in a cell or sector under a givenbandwidth allocation and transmission rate requirement. The unit ofspectral efficiency is the total transmission rate per unit bandwidthwithin a given cell or sector. Obviously, the better the spectralefficiency is, the higher the system capacity will be.

[0004] Traditional wireless Multiple Access Control (MAC) systems, suchas a Frequency Division Multiple Access (FDMA) system, a Time DivisionMultiple Access (TDMA) system, result in system capacity that is limitedby the time-bandwidth (TB) product. It is impossible to increase thenumber of supportable subscribers under these MAC schemes. For example,assume that the basic transmission rate of a subscriber is 1/T samplesper second and the allocated bandwidth is B Hz. Then, the time-bandwidthproduct is BT, which is the maximum number of supportable subscribers.It is impossible to support more than BT subscribers in FDMA and TDMAsystems.

[0005] The situation is completely different under a Code DivisionMultiple Access (CDMA) system where the system capacity only depends onthe Signal-to-Interference Ratio (SIR). Increasing the number ofsubscriber reduces the SIR, thus lowering the transmission rate.However, a subscriber will not be denied radio resource allocation. Inother words, unlike FDMA and TDMA systems, a CDMA system does not have ahard upper bound (i.e. BT) on the number of supportable subscribers.

[0006] The capacity of a CDMA system depends on the interference level.As such, the ability to accurately control the interference level iscritical to the performance and the successful operation of a CDMAsystem. There are four sources of interference in a CDMA system: Thefirst type of interference, or noise, comes from various sources in thelocal environment, which cannot be control by the wireless communicationsystem. The only way to alleviate noise interference is to use a lownoise amplifier. The second type of interference isInter-Symbol-Interference (ISI). The third type of interference isMultiple Access Interference (MAI) that is originated from othersubscribers in the same cell. The forth type of interference is AdjacentChannel or Cell Interference (ACI) that is originated from othersubscribers in the neighboring channel or cell. It is possible to reduceor eliminate ISI, MAI, and ACI by using high performance codes.

[0007] In a CDMA system, each subscriber has his/her own uniqueidentification code. In addition, the subscribers' spread-spectrum codesare orthogonal to each other. The orthogonality requirement is common toall multiple access schemes. If the communication channel is an ideallinear time and frequency non-dispersion system, and the system has highdegree of synchronization, then the subscribers will stay orthogonal toeach other. In reality, the communication channel is not ideal, and itis very difficult to achieve tight synchronization for communicationchannels with time and frequency dispersion. As a result, the ability toachieve orthogonality in a non-ideal communication channel with time andfrequency dispersion is critical to the successful operation of CDMAsystems.

[0008] It is commonly known that a mobile communication channel is atypical random time varying channel, with random frequency dispersion,due to Doppler shift effect, and random time dispersion, due tomulti-path transmission effect. Random frequency dispersion results inthe degradation in time selectivity of the received signal withunexpected fluctuation of the reception power level. Random timedispersion results in the degradation in frequency selectivity, whichresults in the unexpected variation in the reception level within eachfrequency component. This degradation results in reduced systemperformance and significantly lowers the system capacity. In particular,because of the time dispersion of the transmission channel, as a resultof multi-path transmission, different signal paths do not arrive at thereceiver at the same time. This results in the overlapping ofneighboring symbols of the same subscriber and causes Inter SymbolInterference (ISI). On the other hand, the time dispersion of thechannel worsens the multiple access interference. When the relativedelay of signals of different subscribers are zero, any orthogonal codecan achieve orthogonality. However, it is very hard to maintainorthogonality if the relative delay of signals of subscribers is notzero.

[0009] In order to reduce ISI, the auto-correlation of each subscriber'saccess codes must be an ideal impulse function that has all energy atthe origin, nowhere else. To reduce the MAI, the cross-correlationsbetween multiple access codes of different subscribers must be zero forany relative delay. In the terms of orthogonality, each access code mustbe orthogonal to itself with non-zero time delay. The access codes mustbe orthogonal to each other for any relative delay, including zerodelay.

[0010] For simplicity, the value of an auto-correlation function at theorigin is called the main lobe and the values of auto-correlations andcross-correlations at other points are called side lobes. Thecorrelation functions of ideal multiple access codes should have zeroside lobes everywhere. Unfortunately, it is proved in Welch theory thatthere does not exist any ideal multiple access codes in the field offinite elements and even in field of complex numbers. The claim thatideal multiple access codes do not exist, is called the Welch bound.Especially, the side lobes of auto-correlation function and the sidelobes of cross-correlation function are contradicted to each other; asside lobes of one correlation function become small, the side lobes ofthe other correlation function become big. Furthermore, NASA had donebrute force searching, by using a computer, to search for all idealcodes. However, there has not been a breakthrough. Since then, not muchresearch work has been done on the search of the ideal multiple codes.

[0011] NASA searched for the good access codes in the Group codes andthe Welch bound in the sub-fields of complex numbers. Beyond the fieldof complex numbers, the ideal codes could exist. For example, B. P.Schweitzer found an approach to form ideal codes in his Ph.D thesis on“Generalized complementary code sets” in 1971. Later, Leppanen andPentti (Nokia Telecommunication) extended Dr. Schweitser's results inthe mixed TDMA and CDMA system. They broke the Welch bound in the highdimensional space. However, the utilization of frequency is very low andthus there is no practical value. There has not been any application oftheir invention in nearly 30 years. According to their invention, in asystem of N multiple access codes, there requires at least N² basiccodes. Each basic code has length at least N chips. That means it needsN³ chips to support N addresses. For example, when N=128, with 16 QAMmodulation, the coded spectral efficiency is only log₂16×128/128³=2.441×10⁻⁴ bits/Hz. The more access codes, the lower theutilization of the spectral efficiency. This coding methodology remindsus that ideal multiple access codes can be achieved via complementarycode sets. We should, however, avoid that the code length grows too fastwith the required number of multiple access codes.

[0012] In addition, with technique of two-way synchronization, therelative time delay within each access code or between each other in arandom time varying channel will not be greater than the maximum timedispersion of the channel plus the maximum timing error. Assuming thatvalue is Δ second, so long as their correlation functions do not haveany side lobes in a time interval (−Δ,Δ), there are no MAI and ISIbetween the access codes. The time interval that possesses the aboveproperty is called “zero correlation window”. It is obvious that thecorresponding CDMA system will be ideal when the “zero correlationwindow” size is wider than the maximum time dispersion deviation of thechannel, i.e. the time delays among multi-paths of the signal, plus themaximum timing error. At the same time, it is also true that thenear-far effects are no longer effective. The well-known near-fareffects is created by the overlapping of the side lobe of a signalsource that is close to the base station receiver and the main lobe of asignal source that is far away from the base station receiver. The sidelobe over-kills the main lobe, which causes high interference. Theaccurate, complicated and fast power control mechanism has to been usedto overcome the near-far effects so that the energy of signals must bebasically the same at the base station receiver. However, within the“zero correlation window” of the multiple access codes, there are noside lobes in the auto-correlation functions and cross-correlationfunctions under the working condition. The near-far effects no longerexist in the system. The complicated and fast power control mechanismwill become less important and optional.

[0013] Therefore, the distinction between different CDMA systems liesmainly in the selected multiple access codes, i.e. in a good system, ISIand MAT must both be small, otherwise they must be larger.

[0014] Existing CDMA systems have either very low efficiency or havevery short communications distance for example about several hundredmeters or do nothing to MAT and ISI and then all that can be done is toalleviate them by using relatively good multiple access codes.

SUMMARY OF THE INVENTION

[0015] A general object of the invention is a CDMA spread-spectrumsystem for a large area synchronous communications system or a largearea asynchronous communications system.

[0016] An object of the present invention is to provide a new codingmethod for use with a spread-spectrum transmitter and receiver to createa series of spread-spectrum multiple access codes that have the “ZeroCorrelation Window” in their auto-correlation functions andcross-correlation functions. Due to the creation of the “zerocorrelation window”, the fatal near-far effects in traditional CDMAradio communications is solved. The Multiple Access Interference (MAI)and the Inter-Symbol Interference (ISI) is eliminated. A high RFcapacity radio system could be thus created based on the invention.

[0017] According to the present invention, as embodied and broadlydescribed herein, a transmitter and a receiver are provided which use aset of Large-Area Code-Division-Multiple-Access (LA-CDMA) codes. TheLA-CDMA codes are generated from the steps of generating a plurality ofpluses, and generating a plurality of codewords.

[0018] The plurality of pluses has a plurality of intervals between eachof the pulses in the plurality of pulses, respectively. Each pulse ofthe plurality of pulses has an amplitude and a polarity. Each intervalof the plurality of intervals is unequal to other intervals of theplurality of intervals. Only one interval of the plurality of intervalsis an odd number larger than a value of a smallest interval of theplurality of intervals. No value of any interval in the plurality ofintervals is a sum of any two or more values of two or more intervals,respectively, in the plurality of intervals.

[0019] Each codeword in the plurality of codewords is different fromother codewords in the plurality of codewords, by assigning, for eachrespective codeword in the plurality of codewords, a polarity to eachpulse in the plurality of pulses. The plurality of codewords are eitherorthogonal, or bi-orthogonal, or trans-orthogonal. The codewords aregenerated such that a cross-correlation between any two codewords in theplurality of codewords has side-lobes with any of the values of zero,plus amplitude squared, and minus amplitude squared. Any codeword in theplurality of codewords has a zero correlation window in anauto-correlation function and the cross-correlation function with amagnitude equal to the amplitude squared and with a width equal to twotimes the smallest interval of the plurality of intervals.

[0020] Additional objects and advantages of the invention are set forthin part in the description which follows, and in part are obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0022]FIG. 1 illustrates an example of LA-CDMA code groups, with 16codes;

[0023]FIG. 2 is an illustration of the non-periodic auto-correlationfunction for code 1 in FIG. 1;

[0024]FIG. 3 is an illustration of the non-periodic auto-correlationfunction for code 2 in FIG. 1;

[0025]FIG. 4 is an illustration of the non-periodic cross-correlationfunction for code 1 and code 2 in FIG. 1;

[0026]FIG. 5. is an illustration of the non-periodic cross-correlationfunction for code 3 and code 4 of FIG. 1;

[0027]FIG. 6 shows the LA-CDMA codes formed by the relative coding pulsecompressing method;

[0028]FIG. 7 shows the LA-CDMA codes formed by the absolute coding pulsecompressing method;

[0029]FIG. 8 shows the time offsetting and overlapping method to raisethe code's duty ratio;

[0030]FIG. 9 illustrates a base station communicating with severalremote stations;

[0031]FIG. 10 is a block diagram of a spread-spectrum transmitter with acode generator;

[0032]FIG. 11, is a block diagram of a spread-spectrum transmitter witha memory;

[0033]FIG. 12 is a block diagram of a spread-spectrum receiver with aproduct detector;

[0034]FIG. 13 is a block diagram of a spread-spectrum receiver with amatched filter; and

[0035]FIG. 14 shows a diagram of a class receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Reference now is made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

[0037] The present invention provides a new, simpler, clearer and fasterdesign scheme of spread-spectrum multiple access codes. Based on thescheme, both MAI and ISI in the corresponding CDMA system can becontrolled and thus a digital wireless communications system with largecapacity can be constructed.

[0038] Ideal spread-spectrum multiple access codes should satisfy thetwo main conditions: First, each code's auto-correlation function shouldbe an ideal impulse function, i.e. the function should be zeroeverywhere except at the origin. From the view of orthogonality, eachcode should be orthogonal to its own relative time delay version unlessthe relative time delay is zero. Second, the cross-correlation functionbetween any two codes should be zero everywhere. From the view oforthogonality, each code should be orthogonal to all the other codeswith any relative time delay, including the zero delay.

[0039] To elaborate, the auto-correlation values at the origin aredenoted as the main-lobe value, while the auto-correlation values not atthe origin, as well as the cross-correlation values, are denoted asside-lobe values. For an ideal CDMA system, the side-lobe values of allthe auto-correlations and cross-correlations should be zero. For apractical system, however, it is impossible to satisfy that condition.In this case, all that can be done is to try to make the values of theside-lobe values as small as possible, or the main-lobe value toside-lobe value ratio as large as possible, and the number of theside-lobe values as few as possible. As for binary codes, the smallestnon-zero side-lobe value must be +1 or −1.

[0040] Therefore, in some embodiments of the present invention aspread-spectrum multiple access coding scheme controls and reduces theside-lobe values of the auto-correlations and cross-correlations.

[0041] In addition, a random access asynchronous communications systemin which all the user stations' clocks are not controlled by a basestation is desirable because of its simplicity. That system, on theother hand, has a very strict requirement on the spread-spectrummultiple access codes, characteristic. So, some embodiments of thepresent invention give an effective and practical method for such arandom access asynchronous digital communications system.

[0042] The spread-spectrum multiple access codes have basic pulses withnormalized “1” amplitude and width and different polarities. The numberof the basic pulses is determined according to such practical factors asthe number of required users, the number of available pulse compressingcodes, the number of available orthogonal pulse compressing codes, thenumber of available orthogonal frequencies, system bandwidth, thesystem's highest transmission rate, etc. The intervals between the basicpulses on the time axis are all unequal and the basic pulse positions onit are all different, which are both considered together with the basicpulses, polarities when coding.

[0043] Of all the values of the basic pulses' intervals mentioned above,only one is an odd number larger than the smallest interval's value,i.e. the coding length is odd, while the other interval values are alleven. Moreover, any interval's value can not be the sum of any other twoor more interval values.

[0044] According to orthogonality, the spread-spectrum multiple accesscodes mentioned above are sorted into different code groups, in whichthe polarities of the basic pulses are determined by the orthogonalityrequirement and the sequence is sorted according to Hadamard or otherorthogonal matrices, or some kind of bi-orthogonal or trans-orthogonalmatrix.

[0045] The above coding method is a new CDMA spread-spectrum multipleaccess coding scheme for a Large Area Asynchronous WirelessCommunications System or Large Area Synchronous Wireless CommunicationsSystem, and the code groups are named LA-CDMA codes. When doingcorrelation, whether it is auto-correlation or cross-correlation, andwhether it is periodic correlation, or non-periodic correlation, or evenmixed correlation, no two or more basic pulses can meet together besidesat the origin, which ensures that the side-lobes, values are at most +1or −1. Furthermore, there exists a zero correlation window beside theorigin and the main-lobe's value equals the number of basic pulses.Therefore, the side-lobes of the auto-correlations andcross-correlations are controlled and reduced. That is, in thecorresponding CDMA system, both MAI and ISI are controlled, and an idealCDMA system without MAI and ISI can also be realized if the zerocorrelation window were utilized.

[0046] The above principles lead to a new simpler, clearer and fasterdesign scheme of spread-spectrum multiple access codes forspread-spectrum technology and digital multiple access technology. Basedon the scheme, a CDMA system's design can be simplified and largecapacity achieved, so as to solve the contradiction between the growingneed for high capacity and the limited frequency resources.

[0047] Because the side-lobes of the correlations are small and smooth,MAI and ISI are unrelated to the users, access time and thus randomaccess is permitted. Further, so long as the stability of the clocks inthe user stations, transceivers, meets a specific requirement, anasynchronous mode is also permitted.

[0048] In a practical design, to increase the code's duty ratio, theabove mentioned basic pulse also can be formed by pulse compressingcodes, which include one or more binary or m-ary sequences, includingfrequency modulated sequences, or frequency and phase jointly modulatedsequences, or frequency, phase and time jointly modulated sequences,etc.

[0049] In order to raise the transmission data rate or reduce frequencyband-width, or increase the number of multiple access codes number, thecodes can also be time offset and overlapped, where the shift intervalshould be larger than the channel's maximum time dispersion, the maximummulti-path time delay difference. In the case that the shift interval issmaller than the channel's maximum time dispersion, the shifted versionshould be modulated by different orthogonal frequencies.

[0050] In order to raise the code's duty ratio and transmission datarate simultaneously as much as possible, both of the above methods canbe combined, i.e. the basic pulse includes pulse compressing codes,including one or more binary or m-ary sequences, or frequency modulatedsequences, or frequency and phase jointly modulated sequences, orfrequency, phase and time jointly modulated sequences, etc. At the sametime, the codes are time offset and overlapped.

[0051] To further increase the number of multiple access codes, theabove mentioned basic pulse can also be formed by orthogonal pulsecompressing codes, including one or more binary or m-ary sequences, orfrequency modulated sequences, or frequency and phase jointly modulatedsequences, or frequency, phase and time jointly modulated sequences,etc, or the above mentioned basic pulses can be modulated by differentorthogonal frequencies.

[0052]FIG. 1 is a simple LA-CDMA orthogonal code group including 16access code words that can be used by 16 users simultaneously. Each codeword consists of 16 “±” basic pulses. The period of this code group is847. The intervals between pulses are respectively: 38, 40, 42, 44, 46,48, 50, 52, 54, 56, 60, 62, 68, 72, 76 and 39. The polarities of thepulses ensure orthogonality between the codes.

[0053]FIG. 2 and FIG. 3 are non-cyclic auto-correlation curves for code1 and code 2 in FIG. 1, respectively. As shown in FIGS. 4 and 5,cross-correlation functions between other pairs of codes have quitesimilar shapes so that side lobes may equal a value chosen from +1, −1or 0. FIG. 4 is an illustration of the non-periodic cross-correlationfunction for code 1 and code 2 in FIG. 1. FIG. 5. is an illustration ofthe non-periodic cross-correlation function for code 3 and code 4 ofFIG. 1.

[0054] The correlation functions of any other LA-CDMA codes have quitesimilar shapes, and the difference lies in polarities and positions ofside lobes. The features of this code are described as follows:

[0055] 1) Main lobe value of auto-correlation function equals the numberof basic pulses, and also equals the number of orthogonal code words inthe code group.

[0056] 2) There are only three possible values of side lobes in theauto-correlation and cross-correlation function: +1, −1 or 0.

[0057] 3) A zero correlation window in the auto-correlation andcross-correlation function or around the origin exists, and itsmagnitude is equal to plus two times of the minimal interval betweenbasic pulses.

[0058] The LA-CDMA code group that is designed according to thisinvention can control, and in some embodiments minimize, the side-lobevaluess of the auto-correlation and cross-correlation function. Thisenables the CDMA system to control and minimize MAI and ISI,simultaneously.

[0059] Table 1 and Table 2 respectively list minimum periods of LA-CDMAcodes of 16 basic pulses and 32 basic pulses under the conditions ofvarious minimal basic pulse intervals, in order to make it convenientfor selecting such periods. TABLE 1 Periods and Minimum Intervals of16-Pulse LA-CDMA Codes MINIMUM MINIMUM MINIMUM MINIMUM MINIMUM MINIMUMMINIMUM MINIMUM INTERVAL PERIOD INTERVAL INTERVAL INTERVAL PERIODINTERVAL PERIOD 38 847 40 897 42 905 44 923 46 959 48 995 50 1065 521049 54 1081 56 1117 58 1145 60 1179 62 1213 64 1247 66 1269 68 1303 701337 72 1379 74 1395 76 1427 78 1461 80 1495 82 1529 84 1563 86 1587 881619 90 1653 92 1683 94 1715 96 1749 98 1783 100 1811 102 1843 104 1875106 1907 108 1939 110 1971 112 2003 114 2035 116 2067 118 2099 120 2131122 2163 124 2195 126 2227 128 2259 130 2291 132 2323 134 2355 136 2387138 2419 140 2451 142 2483 144 2515 146 2547 148 2579 150 2611 152 2643154 2675 156 2707 158 2739 160 2771 162 2803 164 2835 166 2867 168 2899170 2931 172 2963 174 2995 176 3027 178 3059 180 3091 182 3123 184 3155186 3187 188 3219 190 3251 192 3283 194 3315 196 3347 198 3379 200 3411202 3443 204 3475 206 3507 208 3539 210 3571 212 3603 214 3635 216 3667218 3699 220 3731 222 3763 224 3795 226 3827 228 3859 230 3891 232 3923234 3955 236 3987 238 4019 240 4051 242 4083 244 4115 246 4147 248 4179250 4211 252 4243 254 4275 256 4307

[0060] TABLE 2 Periods and Minimum Intervals of 32-Pulse LA-CDMA CodesMINIMUM MINIMUM MINIMUM MINIMUM MINIMUM MINIMUM MINIMUM MINIMUM INTERVALPERIOD INTERVAL PERIOD INTERVAL PERIOD INTERVAL PERIOD 32 4751 34 446536 4447 38 4489 40 4745 42 4847 44 4889 46 5359 48 4699 50 5225 52 512554 5117 56 5315 58 4725 60 4687 62 4765 64 4423 66 5115 68 5059 70 530772 5299 74 5617 76 4955 78 5133 80 4915 82 5397 84 5499 86 4965 88 529190 5223 92 4837 94 5539 96 5889 98 5373 100 5319 102 5051 104 5331 1065617 108 5991 110 5109 112 5347 114 5383 116 5127 118 4883 120 5211 1225429 124 5737 126 5663 128 5725 130 5623 132 5725 134 5497 136 5323 1385393 140 5465 142 5811 144 5959 146 5893 148 6331 150 6355 152 5943 1546053 156 6075 158 6241 160 6425 162 6475 164 6267 166 6399 168 6517 1706435 172 6491 174 6555 176 6631 178 6665 180 6751 182 6835 184 6839 1866903 188 6971 190 7059 192 7121 194 7295 196 7521 198 7351 200 7543 2027427 204 7521 206 7579 208 7629 210 7689 212 7739 214 7807 216 7875 2187953 220 8031 222 8051 224 8119 226 8173 228 8239 230 8307 232 8375 2348443 236 8499 238 8569 240 8641 242 8743 244 8747 246 8813 248 8881 2508949 252 9011 254 9113 256 9173

[0061] Pulse duty ratio for basic the LA-CDMA code is very low. Forexample, FIG. 1 shows that pulse duty ratio of a 16 basic pulse codewith period of 847 is merely 16/847 (=0.0189). To increase the dutyratio in a practical design, pulse compression codes with goodperformance, such as a Barker sequence or linear frequency modulationcode, can be substituted for each single pulse in the basic code. Inthis way, so long as the received signal passes through a matched filtermatched to this pulse compression code, then the output is the requiredLA-CDMA code. Several solutions for increasing pulse duty ratio includedin this invention are described below.

[0062] Forming an LA-CDMA code by a relative encoding pulse compressionmethod is shown in FIG. 6. A positive pulse in the basic LA-CDMA code isgenerated by two consecutive pulse compression code “B”s with the samepolarity, whereas a negative pulse is generated by a positive and anegative pulse compression code “B”. For instance, considering a16-pulse LA-CDMA code with a period of 847. If a 13-hit Barker sequencewere chosen for the pulse compression code, then the duty ratio of thecode rises to 16×26/847 (=0.4911).

[0063] Forming an LA-CDMA code by an absolute encoding pulse compressionmethod is shown in FIG. 7. A positive pulse in the basic LA-CDMA code isgenerated by a pulse compression code “B”, whereas a negative pulse isgenerated by an inverse, i.e. an inverted polarity “B”, of the pulsecompression code. For instance, still considering a 16-pulse LA-CDMAcode with a period of 847, if a 28-bit pulse compression code werechosen to form a single pulse, then the duty ratio rises to 16×28/847(=0.5289); if a 38-bit pulse compression code were chosen to form asingle pulse, then the duty ratio rises to 16×38/847 (=0.7178).

[0064] Adopting a time-offset overlapped method for increasing the dutyratio is illustrated in FIG. 8, where “a” is the primitive code, “b”,“c”, “d” and “e” are shifted code versions after four shifts,respectively, and “a+b+c+d+e”, is a time-offset overlapped code. Itshould be noted that the time-offset value must be greater than the timedispersion range of the channel; otherwise, either adding a partialresponse equalizer to the receiver in order to reduce time dispersionrange of channel, or adopting various orthogonal frequencies for thetime-offset versions smaller than the time dispersion range of thechannel, should be employed. When synchronization techniques areadopted, it is similar to a TDMA technique in that different shiftversions can be used by different users. Therefore, this can increasethe number of orthogonal codes greatly. In a random access system, eachshifted version of the LA-CDMA code can only be used by one user, butthat method can increase the user's data rate enormously withoutexpanding system bandwidth, or can decrease system bandwidth whileretaining a given data rate.

[0065] Clearly, the time-offset overlapped pulse compression method canalso be employed, which is a mixture the relative encoding compressionmethod of FIG. 6, and the time-offset overlapped method of FIG. 8, or amixture of the absolute encoding compression method of FIG. 7 and thetime-offset overlapped method of FIG. 8. The time-offset overlappedmethod can provide the greatest increase in pulse duty ratio andinformation rate simultaneously, or decrease system bandwidth with datarate unaffected.

[0066] Sometimes it is inconvenient that the maximum number of usersoffered by the basic LA-CDMA code is determined only by the quantity ofbasic pulses, since the more orthogonal codes in the code group, thebetter. Embodiments of this invention may provide three solutions toenlarge the number of users.

[0067] The first solution is to adopt orthogonal pulse compressioncodes. If M pieces of orthogonal pulse compression codes can be found,then M×N orthogonal pulse compression code words can be obtained whenthere are N pulses in an LA-CDMA code. For example, considering a16-pulse LA-CDMA code with a period of 847 and choosing a 32-bitorthogonal code as its pulse compression code, as there are 32orthogonal codes in the 32-bit orthogonal pulse compression code group,there are a total of 16×32 (=512) orthogonal code words.

[0068] The second solution is to adopt orthogonal frequencies. Thesimplest implementation is to utilize a general purpose FDMA/CDMA mixedtechnique. In this way, if M kinds of orthogonal frequencies wereemployed, in which intervals of frequencies are multiples of I/T, here Tis the duration of a pulse in the LA-CDMA code, then M×N orthogonal codewords can be obtained when there are N pulses in the LA-CDMA code. Byintroducing different orthogonal frequencies to different pulses in theLA-CDMA code, especially when the pulse compression method is employed,then the finally acquired code is a compound code of the basic LA-CDMAcode and the chosen pulse compression code. According to compoundencoding theory, the property of a compound code is mainly determined bythe code with worse performance of two elements of the compound code.Thus, when a pulse compression code is chosen poorly, the finalproperties of the auto-correlation function and cross-correlationfunction will worsen. When every pulse is “isolated” by orthogonalfrequencies, the pulse compression code will be “isolated” too,minimizing degradation accordingly and increasing room for choicesgreatly. For instance, still considering a 16-pulse LA-CDMA code with aperiod of 847, when 16 orthogonal frequencies are introduced and a32-bit orthogonal code serves as the pulse compression code, a total of16×16×32 (=8192) orthogonal code words are obtained.

[0069] The third solution is to relax the restriction of orthogonality,i.e. to adopt quasi-orthogonality which uses imperfect orthogonal codes,to increase the number of users. For example, considering an LA-CDMAcode with N pulses, as the order of N basic intervals has no affect onits auto-correlation function and cross-correlation function, the orderof N basic intervals can be arbitrary. When a code group with variousorders of basic intervals is exploited at the same time, the number ofusers will increase enormously. This also can serve as a solution forreducing interference of adjacent service areas or channels.

[0070] CDMA Transmitter with LA-CDMA Codes

[0071]FIG. 9 shows a base station 140 communicating with a plurality ofremote stations 141, 142, 143, 144, 145. The base station 140 and eachof the plurality of remote stations 141, 142, 143, 144, 145 have a CDMAtransmitter and a CDMA receiver, or equivalently, a CDMA transceiver,using the LA-CDMA codes.

[0072] The CDMA codes having a zero correlation window may be used in aspread-spectrum transmitter. In the exemplary arrangement shown in FIGS.10 and 11, representative spread-spectrum transmitters 30, 40 are shown.Data from a data source are processed by transmitter-code means, togenerate a spread-spreactrum-processed signal. The transmitter-codemeans spread-spectrum processes the data with a particular large areacode-division-multiple-access (LA-CDMA) code from a plurality of LA-CDMAcodes. The plurality of LA-CDMA codes have the zero correlation windowwith a respective auto-correlation function. The zero correlation windowhas a value of zero except at an origin. A particular LA-CDMA code ofthe plurality of LA-CDMA codes has a cross-correlation function withother LA-CDMA codes in the plurality of LA-CDMA codes, within the zerocorrelation window. The cross-correlation function has a value of zeroeverywhere inside the zero correlation window.

[0073] The spread-spectrum-processed signal is raised to a carrierfrequency by product device 34, to generate a spread-spectrum signalwith carrier signal cos(ω_(o)t) at a carrier frequency f_(o). Thecarrier signal cos(ω_(o)t) at the carrier frequency f_(o) is from signalsource 35. The output from the product device 34 is filtered by filter36. Filter 36 typically is a bandpass filter, with a bandwidth centeredat the carrier frequency f_(o) and a bandwidth sufficiently wide to passthe spread-spectrum signal. The spread-spectrum signal is amplified byamplifier 37 and radiated by antenna 38.

[0074] In FIG. 10, the transmitter-code means to generate thespread-spectrum-processed signal, includes a code generator 32, productdevice 31 and filter 33. The product device 32 is connected or coupledto the code generator 32 and between the data source and filter 33. Thecode generator 32 generates the particular LA-CDMA code from theplurality of LA-CDMA codes, and any of the other LA-CDMA code in theplurality of LA-CDMA codes. The product device 31 spread-spectrumprocesses the data with the particular CDMA code. The filter 33 filtersthe spread-spectrum-processed signal.

[0075] In FIG. 11, the transmitter-code means to generate thespread-spectrum processed signal, includes a memory 39. The memory 39may be a disk, RAM, or other memory. Memory devices and medium are wellknown in the art. The data includes symbols. In a simple form, thesymbols are 1-bits and 0-bits. Multiple bit symbols, however, may isincluded. In response to a particular symbol of a plurality of symbolsfrom the data source, the memory 39 outputs the particular LA-CDMA codefrom the plurality of LA-CDMA codes stored in the memory 39. The mappingof symbols to LA-CDMA codes preferably is one-to-one.

[0076] The spread-spectrum transmitters 30, 40 of FIGS. 10 and 11,respectively, are representative, and as is well-known in the art, maybe embodied with more or additional features and technology. The presentinvention can be used with more advanced spread-spectrum transmittersthan those depicted in FIGS. 10 and 11.

[0077] Spread-Spectrum Receiver with LA-CDMA Codes

[0078] The exemplary drawings of FIGS. 12 and 13 show two embodiments ofspread-spectrum receivers 50, 70 which may be used to receive aspread-spectrum signal having the particular CDMA code with the zerocorrelation window. The received spread-spectrum signal was transmittedby a spread-spectrum transmitter using the particular LA-CDMA code withthe zero correlation window. The typical spread-spectrum source is anantenna 51, but other sources my be used, such as a cable, or othercommunications channel. Typically a signal source 53 generates thecarrier signal cos(ω_(o)t) at a carrier frequency f_(o). A mixer 52mixes the spread-spectrum signal with the carrier signal cos(ω_(o)t) ata carrier frequency f_(o), for baseband processing. Other frequencies,such as an intermediate frequency, may be used for processing thespread-spectrum signal. The filter 54 filters the spread-spectrum signalat the processing frequency. Such technology is well-known in the art.

[0079] The receiver-code means spread-spectrum processes thespread-spectrum signal with a replica of the particular LA-CDMA codefrom the plurality of LA-CDMA codes. The replica of the particular CDMAcode has a zero correlation window, and an auto-correlation function,within the zero correlation window, having a value of zero except at anorigin. The replica of the particular LA-CDMA code has across-correlation function with other LA-CDMA codes in the plurality ofLA-CDMA codes, within the zero correlation window, having a value ofzero everywhere inside the zero correlation window.

[0080] In FIG. 12, the receiver-code means is embodied as areceiver-code generator 56 a mixer 55 and as filter 57. The mixer 55 iscoupled between the filter 54 and the filter 57, and to the codegenerator 56. The receiver-code generator 56 generates the replica ofthe particular LA-CDMA code from the plurality of LA-CDMA codes. Themixer 55 spread-spectrum processes the spread-spectrum signal at theprocessing frequency with the replica of the particular LA-CDMA code.The filter 57 filters the processed spread-spectrum signal, to outputdata.

[0081] The receiver-code generator 56 generates the replica of theparticular CDMA code with the zero correlation window, and anauto-correlation function, within the zero correlation window, having avalue of zero except at an origin. The replica of the particular LA-CDMAcode has a cross-correlation function with other LA-CDMA codes in theplurality of LA-CDMA codes, within the zero correlation window, having avalue of zero everywhere inside the zero correlation window. Thereceiver-code generator 56 may include a memory for storing the replicaof particular LA-CDMA code, or the entire plurality of replicas ofLA-CDMA codes. Other signal generating techniques, including switchingand logic circuitry, are well-known in the art, may be used forgenerating one or all of the LA-CDMA codes.

[0082] In FIG. 13, the received-code means is embodied as a matchedfilter 71. The matched filter has an impulse response, matched to theparticular CDMA code of the spread-spectrum signal being received by thespread-spectrum receiver 70. Preferably, the matched filter 71 is aprogrammable matched filter, which, by control of processor 72, canchange the impulse function of the matched filter 71. The matched filter71 may include a two-stage, or multi-stage matched filter, depending onsystems requirements and design criteria. The matched filter 71 may be asurface-acoustic-wave (SAW) device. In response to detecting theparticular CDMA code embedded in the received spread-spectrum signal,the matched filter 71 outputs the particular symbol of the plurality ofsymbols. The particular symbol typically might be the 1-bit and the0-bit.

[0083]FIG. 14 is a block diagram of a receiver 10 for a LA-CDMA randomaccess code division multiple access wireless system exploiting oneembodiment of this invention. This system adopts 16-pulse LA-CDMA codesand 4 orthogonal frequencies, and can accommodate 64 users signalingsimultaneously. The basic structures of a transmitter and a receiver maybe readily ascertained once the information basic formula and modulationmode are decided. Detailed implementations may entail some modificationaccording to practical situations. For example, a receiver can berealized either by a matched filter or by a correlator. They bothimplement correlation operations, and essentially have no distinction.In these cases, a transmitter must generate the required modulatedwaveforms that can be demodulated by computation. Generally, thereceiver's structure is comparatively simple, such that a wirelesstelecommunication engineer can design it in the light of basic modulatedsignal waveform.

[0084] The 16-pulse LA-CDMA code with a period of 847 shown in FIG. 1 isadopted as a multiple access code in this system. Moreover, it utilizes4 orthogonal frequencies, and each frequency's interval is thereciprocal of the basic pulse's duration. A relative coding pulsecompression method is employed to generate the basic LA-CDMA code, withmodulation performed using binary phase-shift keying (“BPSK”), and witha pulse compression code of a 13-bit Barker sequence, which is 1 1 1 1 1−1 −1 1 1 −1 1 −1 1.

[0085] Users are permitted to transmit using random access, and toreceive by a matched filter. FIG. 13 depicts a receiver's block diagramfor a certain orthogonal frequency. An analog signal from anintermediate frequency amplifier is converted to a digital signal by ananalog to digital converter 11. The system 10 detects a 13-bit Barkersequence using a pulse shape matched filter 12 that includes a 13-bitdigital tap delay line 14, multipliers 16 with a 13-bit stage shiftregister 15, a low pass filter 18 and a weak signal detector or smallsignal depressor 20. An 808-bit digital tap delay line 22 and anadditional logic circuit 24, which is another part of the receiver, forma pulse position matched filter 26.

[0086] The pulse shape matched filter 26 forms pulses of the basicLA-CDMA code, while the pulse position matched filter implements a matchoperation an the LA-CDMA code. A pulse position matched filter canimplement match operations on 16 orthogonal LA-CDMA code simultaneously.FIG. 1 illustrates an example of LA-CDMA code groups (with 16 codes).

[0087] It will be apparent to those skilled in the art that variousmodifications can be made to the large area wireless CDMA method andapparatus of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the large area wireless CDMAmethod and apparatus provided they come within the scope of the appendedclaims and their equivalents.

I claim:
 1. A method for generation a set of Large-AreaCode-Division-Multiple-Access (LA-CDMA) codes, comprising the steps of:generating a plurality of pluses with a plurality of 0.5 intervalsbetween each of the pulses in the plurality of pulses, respectively,each pulse of the plurality of pulses having an amplitude and apolarity, with each interval of the plurality of intervals unequal toother intervals of the plurality of intervals, with only one interval ofthe plurality of intervals being an odd number larger than a value of asmallest interval of the plurality of intervals, without a value of anyinterval in the plurality of intervals being a sum of any two or morevalues of two or more intervals, respectively, in the plurality ofintervals; and generating a plurality of codewords from the plurality ofpulses, with each codeword in the plurality of codewords different fromother codewords in the plurality of codewords, by assigning, for eachrespective codeword in the plurality of codewords, a polarity to eachpulse in the plurality of pulses, such that the plurality of codewordsare any of orthogonal, bi-orthogonal and trans-orthogonal, and such thata cross-correlation between any two codewords in the plurality ofcodewords has side lobes with any of the values of zero, plus amplitudesquared, and minus amplitude squared, and any codeword in the pluralityof codewords has a zero correlation window in an auto-correlationfunction and the cross-correlation function with a magnitude equal tothe amplitude squared and with a width equal to two times the smallestinterval of the plurality of intervals.
 2. The method as set forth inclaim 1, further including the step of spread-spectrum processing eachpulse in the plurality of pulses, by a pulse compression code.
 3. Themethod as set forth in claim 1, wherein the step of spread-spectrumprocessing further includes the step of spread-spectrum processing eachpulse of the plurality of pulses, by a Barker sequence.
 4. The method asset forth in claim 1, wherein the step of spread-spectrum processingfurther includes the step of spread-spectrum processing each pulse ofthe plurality of pulses, by linear frequency modulation.
 5. The methodas set forth in claim 1, further including the step of time-offsettingand overlapping each codeword in the plurality of codewords.
 6. Themethod as set forth in claim 5 further including the step of adoptingdifferent orthogonal modulating frequencies for different versions ofthe time-offset codewords in the plurality of codewords.
 7. Atransmitter for transmitting a spread-spectrum signal from a set ofLarge-Area Code-Division-Multiple-Access (LA-CDMA) codes, comprising:transmitter-code means for generating a plurality of pluses with aplurality of intervals between each of the pulses in the plurality ofpulses, respectively, each pulse of the plurality of pulses having anamplitude and a polarity, with each interval of the plurality ofintervals unequal to other intervals of the plurality of intervals, withonly one interval of the plurality of intervals being an odd numberlarger than a value of a smallest interval of the plurality ofintervals, without a value of any interval in the plurality of intervalsbeing a sum of any two or more values of two or more intervals,respectively, in the plurality of intervals; said transmitter-code meansfor generating a plurality of codewords from the plurality of pulses,with each codeword in the plurality of codewords different from othercodewords in the plurality of codewords, by assigning, for eachrespective codeword in the plurality of codewords, a polarity to eachpulse in the plurality of pulses, such that the plurality of codewordsare any of orthogonal, bi-orthogonal and trans-orthogonal, and such thata cross-correlation between any two codewords in the plurality ofcodewords has side lobes with any of the values of zero, plus amplitudesquared, and minus amplitude squared, and any codeword in the pluralityof codewords has a zero correlation window in an auto-correlationfunction and the cross-correlation function with a magnitude equal tothe amplitude squared and with a width equal to two times the smallestinterval of the plurality of intervals; said transmitter-code means forprocessing data to generate a spread-spectrum-processes signal; andtransmitter means for raising the spread-spectrum-processes signal to acarrier frequency, and for transmitting the spread-spectrum-processedsignal as a spread-spectrum signal using radio waves.
 8. The transmitteras set forth in claim 7, with said transmitter-code means furtherincluding means for spread-spectrum processing each pulse in theplurality of pulses, by a pulse compression code.
 9. The transmitter asset forth in claim 7, with said transmitter-code means further includingmeans for spread-spectrum processing each pulse of the plurality ofpulses, by a Barker sequence.
 10. The transmitter as set forth in claim1, claim 7, with said transmitter-code means further including means forspread-spectrum processing each pulse of the plurality of pulses, bylinear frequency modulation.
 11. The mtransmitter as set forth in claim7, with said transmitter-code means further including means fortime-offsetting and overlapping each codeword in the plurality ofcodewords.
 12. The transmitter as set forth in claim 7, with saidtransmitter-code means further including means for adopting differentorthogonal modulating frequencies for different versions of thetime-offset codewords in the plurality of codewords.
 13. A receiver forreceiving a spread-spectrum signal generated from a set of Large-AreaCode-Division-Multiple-Access (LA-CDMA) codes, comprising: receiver-codemeans for generating a plurality of pluses with a plurality of intervalsbetween each of the pulses in the plurality of pulses, respectively,each pulse of the plurality of pulses having an amplitude and apolarity, with each interval of the plurality of intervals unequal toother intervals of the plurality of intervals, with only one interval ofthe plurality of intervals being an odd number larger than a value of asmallest interval of the plurality of intervals, without a value of anyinterval in the plurality of intervals being a sum of any two or morevalues of two or more intervals, respectively, in the plurality ofintervals; said receiver-code means for generating a plurality ofcodewords from the plurality of pulses, with each codeword in theplurality of codewords different from other codewords in the pluralityof codewords, by assigning, for each respective codeword in theplurality of codewords, a polarity to each pulse in the plurality ofpulses, such that the plurality of codewords are any of orthogonal,bi-orthogonal and trans-orthogonal, and such that a cross-correlationbetween any two codewords in the plurality of codewords has side lobeswith any of the values of zero, plus amplitude squared, and minusamplitude squared, and any codeword in the plurality of codewords has azero correlation window in an auto-correlation function and thecross-correlation function with a magnitude equal to the amplitudesquared and with a width equal to two times the smallest interval of theplurality of intervals; and said receiver-code means for despreading areceived spread-spectrum signal using a particular LA-CDMA code.
 14. Thereceiver as set forth in claim 13, with said receiver-code means furtherincluding means for spread-spectrum processing each pulse in theplurality of pulses, by a pulse compression code.
 15. The receiver asset forth in claim 13, with said receiver-code means further includingmeans for spread-spectrum processing each pulse of the plurality ofpulses, by a Barker sequence.
 16. The receiver as set forth in claim 13,with said receiver-code means further including means forspread-spectrum processing each pulse of the plurality of pulses, bylinear frequency modulation.
 17. The receiver as set forth in claim 13,with said receiver-code means further including means fortime-offsetting and overlapping each codeword in the plurality ofcodewords.
 18. The receiver as set forth in claim 13, with saidreceiver-code means further including means for adopting differentorthogonal modulating frequencies for different versions of thetime-offset codewords in the plurality of codewords.
 19. A receiver forreceiving a spread-spectrum signal generated from a set of Large-AreaCode-Division-Multiple-Access (LA-CDMA) codes, comprising: a matchedfilter having an impulse function matched to a plurality of pluses witha plurality of intervals between each of the pulses in the plurality ofpulses, respectively, each pulse of the plurality of pulses having anamplitude and a polarity, with each interval of the plurality ofintervals unequal to other intervals of the plurality of intervals, withonly one interval of the plurality of intervals being an odd numberlarger than a value of a smallest interval of the plurality ofintervals, without a value of any interval in the plurality of intervalsbeing a sum of any two or more values of two or more intervals,respectively, in the plurality of intervals; said matched filter havingan impulse function matched to a plurality of codewords from theplurality of pulses, with each codeword in the plurality of codewordsdifferent from other codewords in the plurality of codewords, byassigning, for each respective codeword in the plurality of codewords, apolarity to each pulse in the plurality of pulses, such that theplurality of codewords are any of orthogonal, bi-orthogonal andtrans-orthogonal, and such that a cross-correlation between any twocodewords in the plurality of codewords has side lobes with any of thevalues of zero, plus amplitude squared, and minus amplitude squared, andany codeword in the plurality of codewords has a zero correlation windowin an auto-correlation function and the cross-correlation function witha magnitude equal to the amplitude squared and with a width equal to twotimes the smallest interval of the plurality of intervals; and saidmatched filter for despreading a received spread-spectrum signal using aparticular LA-CDMA code.
 20. The receiver as set forth in claim 19, withsaid matched filter further including an impulse function forspread-spectrum processing each pulse in the plurality of pulses, by apulse compression code.
 21. The receiver as set forth in claim 19, withsaid matched filter further including an impulse function forspread-spectrum processing each pulse of the plurality of pulses, by aBarker sequence.
 22. The receiver as set forth in claim 19, with saidmatched filter further including an impulse function for spread-spectrumprocessing each pulse of the plurality of pulses, by linear frequencymodulation.
 23. The receiver as set forth in claim 19, with said matchedfilter further including means for time-offsetting and overlapping eachcodeword in the plurality of codewords.
 24. The receiver as set forth inclaim 19, with said matched filter further including an impulse functionfor adopting different orthogonal modulating frequencies for differentversions of the time-offset codewords in the plurality of codewords.